This invention relates to treating radioactive and hazardous wastes. More particularly, this invention relates to a method for treating air filters contaminated with radioactive and/or hazardous materials such that these radioactive and hazardous materials are efficiently and inexpensively removed.
High Efficiency Particulate Air (HEPA) filters are replaceable extended-media dry-type filters in a rigid frame having a maximum particle collective efficiency of 99.97% for a 0.3 xcexcm particle and a maximum clean filter pressure drop of 2.54 cm water gauge when tested at rated air flow capacity. HEPA filter media are typically a mixture of fire retardant glass fibers, special acid resistant materials (KEVLAR/NOMAX), and a chemical binder, however, stainless steel HEPA filters are also in use. Corrugated, overlapped filter media are sealed to the edges of the filter housing with high temperature resistant silicone. The typical filter housing is made of 14 gauge 300 series austenitic stainless steel. Most filters have a plastic mesh on the top and a stainless steel screen on the bottom to ensure that the filter media remain intact.
HEPA filters are used to clean air for operations involving a variety of hazardous materials from asbestos to radionuclides. HEPA filters are widely used, and their use will likely increase in the future, both in waste clean-up and in applications such as indoor air quality and disposal of old fluorescent lamps. HEPA filters are used in medical, military, electronic, and industrial applications where clean or super-clean air is essential to the work.
An example of how HEPA filters are used comes from the New Waste Calciner Facility (NWCF) at the Idaho Nuclear Technology and Engineering Center. This facility was originally designed to process spent nuclear fuel by dissolving the fuel and extracting the reusable uranium. The waste is calcined to form a dry powder and put into concrete storage bins for indefinite storage. NWCF operations involve the handling of fission products, transuranic (TRU) materials (i.e., alpha-emitting radionuclides with atomic numbers greater than 92 and half-lives greater than 20 years), and hazardous materials. HEPA filters are used in the off-gas streams to prevent these materials from entering and contaminating the environment. As a result, many HEPA filters contaminated with radioactive (5-120 R/hr (xcex2/xcex3)), transuranic (TRU content  greater than 600 nCi/g has been observed), and toxic metal constituents (6-12 mg Cd/g, 2-4 mg Cr/g, and 3-6 mg Hg/g) have been generated. R. I. Donovan et al., NWCF HEPA Filter Leaching Demonstration Results (1984); C. W. McCray and K. N. Brewer, HEPA Filter Leach System Technical Basis Report, WINCO-1096 (1992); A. C. Chakravartty, HEPA Filter Leaching Concept Validation Trials at the Idaho Chemical Processing Plant, INEL-95/0182 (1995). Currently there is no disposal facility that will accept such contaminated HEPA filters. Hence, a HEPA filter leach system was designed to lower radiation and contamination levels and reduce cadmium, chromium, and mercury concentrations on spent HEPA filter media to below disposal limits set by the Resource Conservation and Recovery Act (RCRA). The treated HEPA filters are then disposed of as low-level radioactive waste.
The leach system currently in use for treating the NWCF HEPA filters has been approved by the EPA and the State of Idaho. According to the approved procedure, leaching of each filter requires three 60-gallon (227.1 liters) leach cycles using 1 to 3 M nitric acid, followed by two 60-gallon water rinses. This procedure generates 300 gallons (1135.6 liters) of dilute nitric acid waste per filter. Nitric acid is used in the leaching process because of its ability to dissolve calcine, remove radioisotopes, and extract RCRA metals from the filters. Validation tests of this leaching system resulted in reduction of radioactivity from 5-120 R/r (xcex2/xcex3) to 30-90 mR/hr (xcex2/xcex3), while TRU levels dropped from greater than 600 nCi/g to 0.4-3.6 nCi/g. Cadmium, chromium, and mercury concentrations on spent HEPA filter media were successfully reduced to below disposal limits set by the RCRA. Since the validation tests, 139HEPA filters classified as TRU mixed waste have been processed and disposed of as low-level radioactive waste.
The existing HEPA filter leaching system performed well in the past, however, as the concentration of mercury in the calciner off-gas system has increased in recent years, more mercury has been deposited on HEPA filters. The leaching system has difficulty removing the higher concentrations of mercury from these HEPA filters. The filter media and the trapped calcine particles are confined in a heavy filter housing that contributes to poor mixing zones around the edges of the filter housing. Other inefficiencies in the existing filter leaching system that result in poor solubility and mass transfer of the contaminants from the HEPA filter media include low media permeability, channeling of the liquid through cracks and tears in the filter media, and liquid retention between leach and rinse cycles.
In addition to the leaching method described above, other methods have been reported in the literature for treating HEPA filters, including compaction, supercompaction, incineration, vitrification or melting, dissolution of the filter medium, and macroencapsulation in cement.
Compaction and supercompaction are relatively inexpensive methods for reducing waste volume, but they do not remove any contaminants from the filters, stabilize radionuclides, or qualify as valid RCRA treatments for characteristic metals. Controlling and containing the spread of contamination for these processes are also a concern.
Incineration is an excellent treatment process in terms of volume reduction for HEPA filters with wood housings. D. L. Zeigler and A. J. Johnson, Disposal of HEPA Filters by Fluidized Bed Incineration, RFP-2768 (1978); D. L. Zeigler and A. J. Johnson, Disposal of HEPA Filters by Fluidized Bed Incineration, Proc. 15th Nucl. Air Cleaning Conf., Boston, Mass. 5 (1978). This process meets RCRA treatment standards for waste that contains both listed and characteristic RCRA constituents assuming the final grouted ash waste form can pass toxicity characterization leaching procedure (TCLP). The final waste form, however, would still be considered an RCRA listed waste if the original waste had any RCRA listed waste in it. Incineration processes are among the most expensive treatment options and have problems associated with treating waste containing volatile metals. Where the housings for HEPA filters are stainless steel, only the polymer coating on the fiberglass media and seal material burn. Therefore, incineration is not practical or useful for volume reduction. Moreover, the off-gas requires expensive treatment, because many of the HEPA filters contain volatile mercury.
Vitrification or melting is the best process available in terms of overall volume reduction. This process is expected to meet RCRA treatment standards for waste that contains characteristic RCRA constituents, since vitrified waste forms typically perform well in passing TCLP. Vitrification or melting typically has problems associated with treating waste containing volatile metals. Hence, scrubbing of the off-gas is required because many of the HEPA filters contain volatile mercury compounds. This process does not meet RCRA requirements for treating listed waste. Moreover, melting is both an expensive and energy-intensive process.
Dissolution is a process wherein the filter media are removed from the filter housings and dissolved in an appropriate solvent, such as hydrofluoric acid (HF). D. E. Clark, Use of Sulfuric-Nitric Acid for the Recovery of Plutonium from HEPA Filters, HEDL-TME-78-67 (1978); F. M. Scheitlin and W. D. Bond, Recovery of Plutonium from HEPA Filters by Ce (IV): Promoted Dissolution of PUO2 and Recycle of the Cerium Promoter, ORNL/TM-6802 (1980); R. E. Lueze et al., Recovery of Plutonium from HEPA Filters by Ce(IV)-Promoted Dissolution of PuO2 and Recycle of the Cerium Promoter, Proc. Int""l Conf. Nucl. Waste Transmutation 33 (1980); K. N. Brewer and J. A. Murphy, HEPA Filter Dissolution Process, U.S. Pat. No. 5,288,434 (1994); K. N. Brewer and J. A. Murphy, Hydrofluoric Acid Dissolution of Spent Radioactive and Hazardous HEPA Filters, WINCO-11814 (1992); K. N. Brewer and J. A. Murphy, Hydrofluoric Acid Dissolution of Spent Radioactive and Hazardous HEPA Filters, Proc. Spectrum ""92 Nucl. And Hazardous Waste Management Int""l Topical Meeting 12 (1992). The process not only dissolves the fiberglass filter media, but also destroys any organic compounds present. Dissolution of filter media is a relatively inexpensive process; costs are comparable to the filter leaching system described above. However, the process produces a secondary corrosive liquid waste stream that requires further treatment. This secondary waste is difficult to treat due to high dissolved solid and salt content.
Macroencapsulation of the filters in cement is a relatively inexpensive process that stabilizes the characteristic metal and radionuclide contamination by encapsulating the whole HEPA filter in cement. T. Koya and F. Toshimasa, Method for Treating HEPA Filter, JP6075095 (1994). This treatment meets RCRA standards for treating characteristic metals, assuming the final waste form can pass TCLP. However, this treatment is not considered adequate if the waste contains any RCRA listed waste. Further, the process results in a significant volume increase. Also, disposal options for the final waste form are limited. It is likely that some of the waste would fall in the 10 to 100 nCi/g range for TRU waste, for which there is no permitted facility that would accept the final waste form.
In view of the foregoing, it will be appreciated that providing a method for treating HEPA filters contaminated with radioactive and/or hazardous materials, wherein the method is effective and inexpensive, minimizes/eliminate the use of strong acids, results in a reduced volume of treated material, and results in a minimal amount of secondary waste, would be a significant advancement in the art.
It is a feature of an illustrative embodiment of the present invention to provide a method for treating contaminated HEPA filter media that is relatively inexpensive, improves circulation of fluid to contaminated filter media for improved contamination removal, and minimizes/eliminates strong acids.
It is also a feature of an illustrative embodiment of the invention to provide a method for treating contaminated HEPA filter media that includes a pretreatment step for separating loose contaminants, thereby requiring less contamination to be removed in subsequent steps.
It is another feature of an illustrative embodiment of the invention to provide a method for simultaneously processing multiple filters, thereby resulting in processing of filters at a greater rate.
It is still another feature of an illustrative embodiment of the invention to provide a method for treating contaminated HEPA filters that permits compaction of filter housings and treated filter media, thereby resulting in a reduced volume of treated material.
It is yet another feature of an illustrative embodiment of the invention to provide a method for treating HEPA filters wherein the processing fluid is recycled, resulting in a minimal amount of secondary waste.
These and other features can be addressed by providing a method for reducing contamination with radioactive and hazardous materials of a HEPA filter comprising a filter medium and housing in which the filter medium is contained comprising:
removing loose particles and solids from the HEPA filter;
removing the filter medium from the housing;
decontaminating the housing; and
after removing the filter medium from the housing, processing the filter medium in a liquid medium for removing trapped particles;
disposing of the filter medium from which the particles have been removed as non-RCRA waste;
collecting the particles from the liquid medium, stabilizing the collected particles, and disposing of the stabilized particles; and
recycling the liquid medium after the particles are collected therefrom.